Trellis sequence detector for detecting a quadrature servo signal in disk recording systems

ABSTRACT

A trellis sequence detector is disclosed for detecting a quadrature servo signal in optical disk storage devices. The quadrature servo signal is generated in discrete time and comprises two sinusoidal signals phase offset by 90 degrees. The sampling frequency of the quadrature signal is controlled relative to the track crossing velocity so as to produce a fixed number of samples per period of the sinusoid (i.e., per track crossing). In the embodiment disclosed herein, the sampling of the sinusoidal signals is controlled such that there are eight samples per track crossing. The eight samples are associated with a state transition diagram and corresponding trellis diagram which define the operation of a trellis sequence detector. The trellis sequence detector processes the actual samples of the quadrature signal to determine an estimated quadrature sequence closest to the actual samples of the quadrature signal. In this manner, spurious pulses in the actual quadrature signal are filtered out which significantly enhances the performance of the servo system.

CROSS REFERENCE TO RELATED APPLICATIONS AND PATENTS

This patent is related to other co-pending U.S. patent applications,namely application Ser. No. 08/912,916 entitled "DIFFERENTIAL PHASEERROR DETECTOR USING DUAL ARM CORRELATION FOR SERVO TRACKING IN ANOPTICAL DISK STORAGE DEVICE", and Ser. No. 08/989,272 entitled"GENERATING A QUADRATURE SEEK SIGNAL FROM A DISCRETE-TIME TRACKING ERRORSIGNAL AND A DISCRETE-TIME RF DATA SIGNAL IN AN OPTICAL STORAGE DEVICE."This patent is also related to U.S. Pat. No. 5,291,499 entitled "METHODAND APPARATUS FOR REDUCED-COMPLEXITY VITERBI-TYPE SEQUENCE DETECTORS."The foregoing U.S. patent applications and U.S. patent are herebyincorporated by reference.

FIELD OF INVENTION

The present invention relates to optical disk storage systems,particularly to a trellis sequence detector for detecting a quadratureservo signal for use in counting track crossings during a seekoperation.

BACKGROUND OF THE INVENTION

Optical disk drives, such as compact disks (CDs) and digital video disks(DVDs), are commonly used for storing large amounts of digital data on asingle disc for use in audio/video or computer applications, and thelike. The data on an optical disc is typically recorded as a series of"pits" arranged in tracks, where the length of the pit determines thepresence of a digital "0" bit or a "1" bit. To read this recorded data,a servo system focuses a laser beam onto the surface of the disc suchthat the characteristics of the reflected beam allow detection of thedata pits.

To this end, the servo system performs four operations: (1) a captureoperation to "pull-in" the initial focus position, (2) a seek operationto move the beam radially over the surface of the disk to a desiredtrack, (3) a centerline tracking operation to maintain the beam over thecenterline of the selected track while reading the recorded data, and(4) a focus tracking operation to maintain proper focus as the diskspins over the beam.

Conventional optical disk drives use a head assembly comprised of alaser diode for generating the laser beam which is focused onto thesurface of the optical disk through an objective lens. FIG. 1illustrates a typical optical head assembly, the operation of which iswell known by those skilled in the art. A laser diode 1 produces a lightbeam 2 which passes through a polarization beam splitter 3 and acollimator lens (not shown). The light beam 2 is then reflected by aprism 4, through an object lens (OL) 5, and onto the surface of theoptical disk (not shown). The beam 2 reflects off the optical disc,again passes through the OL 5, and then reflects off prism 4 back towardthe polarization prism 3 which deflects the beam 2 onto a four-quadrantphotodetector 6. The signals output by the four-quadrant photodetector 6are used to generate a focus error signal for focusing the OL 5 and atracking error signal for tracking the centerline of the selected track.The four-quadrant photodetector 6 also generates an RF read signal forreading the recorded data.

In order to position the read head over a selected track during a seekoperation, the entire sled assembly 8 slides radially along a lead screw9 underneath the optical disc until the read head is positioned near thedesired track. This coarse positioning (or coarse seeking) isaccomplished by rotating the lead screw 9 in a clockwise orcounterclockwise direction. Once near the selected track, OL voice coilmotors (VCMs) (10A,10B) rotate an OL carriage unit 11 about a plastichinge 12 in a "fine seeking" operation until the OL 5 is positioneddirectly over the desired track. Then, as the disk rotates and the trackpasses under the read head, the OL VCMs (10A,10B) perform fineadjustments in a "tracking" operation in order to maintain the positionof the OL 5 over the centerline of the selected track as information isread from the disc.

The OL VCMs (10A,10B) also move the OL carriage unit 11 up and down inthe direction shown in order to "capture" and "track" the OL 5 focusposition. For focus capture and focus tracking the four-quadrantphotodetector 6 generates an astigmatic focus error signal indicative ofthe distance between the OL 5 and the optical disc. At the beginning ofa capture operation, the OL carriage unit 11 is initially positionedsufficiently away from the disc so that it is out of focus. Then the OLVCMs (10A, 10B) slowly move the OL carriage unit 11 toward the disc withthe focus servo loop open until the quadrant photodetector 6 indicatesthat the OL 5 is within its focus pull-in range. Once within the pull-inrange, the focus servo loop is closed and the initial focus point iscaptured. Thereafter, the OL VCMs (10A,10B) track the in-focus positionin response to the astigmatic focus error signal as the read head seeksto selected tracks and reads data from the disc.

Several methods have been employed in the prior art for generating thetracking error signal used to maintain the optical transducer over thecenterline of the selected track during a read operation. One method,referred to as differential phase detection (DPD), measures the phaseoffset between a pair of diagonal signals generated by the four-quadrantphotodetector 6 to determine the position error as illustrated in FIG.2A-2C. It should be noted that other types of photodetectors, such as aholographic photodetector, could be used to generate the diagonalsignals. FIG. 2A shows three situations when the pit image is detectedby the photodetector 6: left of center, at the center, and right ofcenter. FIG. 2B shows the resulting diagonal signals generated by addingthe (A+C) quadrants and the (B+D) quadrants, where the phase differencebetween these signals represents the position error. The position errorsignal (PES) is computed by converting the diagonal signals (A+C) and(B+D) into polarity square waves, as shown in FIG. 2C, and thenextracting the offset or time difference between the square waves. Thetime difference is then integrated to generate the tracking error signalapplied to the OL VCMs (10A,10B).

A problem with the above-described prior art method for generating thetracking error signal is that the differential phase error detector isdependent on the spectral content of the data being read from the disk.Thus, the randomness of the recorded data results in gain variance inthe servo tracking loop; to compensate for the gain variance, thetracking servo loop is normally operated at a low (sub-optimal)bandwidth. Another drawback of prior art differential phase errordetectors is a phenomenon known as "lens shift", an effective skewintroduced into the diagonal signals due to generating the positionerror signal in continuous time.

During the seek operation, the position of the light beam on the disk isestimated by detecting track crossings. A track counter circuitincrements/decrements a counter when it detects that the light beam hascrossed a track. When the counter reaches a target value, the light beamwill be on or near the target track. In the prior art, track crossingsare typically estimated by detecting zero crossings or peaks in thecontinuous time tracking error signal (TES). The problem with thistechnique, of course, is that noise in the continuous TES signal mayintroduce spurious pules which can result in misdetected or falselydetected track crossings. Various prior art methods have been employedto reduce errors in the track crossing detector. For example, U.S. Pat.No. 5,199,017 employs hysteresis to prevent the detection of twoconsecutive positive or negative peaks in the TES, and U.S. Pat. No.5,457,671 employs a "window signal" wherein TES zero crossings aredetected only within a predetermined window, and if not detected thenone is inserted.

In another prior art method for seeking a quadrature signal is generatedfrom the TES signal and the data signal (i.e., the RF baseband signal)for use in counting track crossings as well as for determining thehead's radial direction as it moves across the disk (i.e., radially inor out). Determining the radial direction of movement is important whenthe seek velocity is below the run-out velocity where the eccentricityof the disk can cause the direction of track crossings to actuallyreverse direction. The prior art methods for generating a quadraturesignal from the TES and RF baseband signals are analog in nature. Theabove referenced copending U.S. patent application entitled "GENERATINGA QUADRATURE SEEK SIGNAL FROM A DISCRETE-TIME TRACKING ERROR SIGNAL ANDA DISCRETE-TIME RF DATA SIGNAL IN AN OPTICAL STORAGE DEVICE" discloses amethod for generating the quadrature signal in discrete time whichovercomes drawbacks inherent in the prior art analog methods. Thepresent invention discloses further advantages and improvements to thispreviously disclosed discrete time quadrature signal generator.

SUMMARY OF THE INVENTION

A trellis sequence detector is disclosed for detecting a quadratureservo signal in optical disk storage devices. The quadrature servosignal is generated in discrete time and comprises two sinusoidalsignals phase offset by 90 degrees. The sampling frequency of thequadrature signal is controlled relative to the track crossing velocityso as to produce a fixed number of samples per period of the sinusoid(i.e., per track crossing). In the embodiment disclosed herein, thesampling of the sinusoidal signals is controlled such that there areeight samples per track crossing. The eight samples are associated witha state transition diagram and corresponding trellis diagram whichdefine the operation of a trellis sequence detector. The trellissequence detector processes the actual samples of the quadrature signalto determine an estimated quadrature sequence closest to the actualsamples of the quadrature signal. In this manner, spurious pulses in theactual quadrature signal are filtered out which significantly enhancesthe performance of the servo system.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects and advantages of the present invention willbe better understood by reading the following detailed description ofthe invention in view of the drawings, wherein:

FIG. 1 shows a conventional optical head assembly positioned radiallyover the disk by means of a lead screw and optical carriage, where thetracking error signal (TES) is computed using the output of afour-quadrant photodetector.

FIG. 2A-2C illustrate operation of a conventional differential phaseerror detector for computing the position error during tracking.

FIG. 3A shows the waveforms according to the dual arm correlationtechnique of the present invention for generating the position errorsignal during tracking.

FIG. 3B shows the waveforms according to the adaptive dual armcorrelation technique of the present invention which is insensitive tothe frequency content of the recorded data and provides an increase inrange to plus or minus one-half a track.

FIG. 4 is a block diagram of the adaptive length dual arm correlator andquadrature signal sequence detector of the present invention.

FIG. 5 shows details of the bandpass and lowpass filters of FIG. 4.

FIG. 6 shows the quadrature sinusoidal position error signals (PES) andtarget samples when the correlation length is adjusted to generate eightsamples per track crossing.

FIG. 7A is a state transition diagram of the trellis sequence detectorfor detecting a track crossing signal from the quadrature PES signals.

FIG. 7B shows part of a trellis diagram which corresponds to the statetransition diagram of FIG. 7A.

FIG. 8 is a block diagram of the trellis sequence detector whichoperates according to the state transition diagram of FIG. 7A.

FIG. 9A and FIG. 9B show the staircased, sawtooth waveforms output bythe trellis sequence detector depending on whether the storage system isexecuting a forward or reverse seek.

FIG. 10 shows details on the preferred embodiment for sampling thefour-quadrant photodetector signals and for generating the diagonalsignals S1 and S2.

FIG. 11 shows details of the positive and negative correlators CorrP(Δ)and CorrN(Δ) which compute their respective correlations by addingadjacent correlations at Δ+ and Δ-.

FIG. 12 is a flow diagram showing how the correlation offset Δ isadaptively adjusted to maximize the positive or negative correlations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Overview

The present invention generates a quadrature signal for detecting trackcrossings by sampling the diagonal signals from a four-quadrantphotodetector, correlating the sampled diagonal signals to generatepositive and negative correlation signals CorrP and CorrN, adding andsubtracting the correlation signals to generate a first and secondposition error signals that are in quadrature, and detecting a trackcrossing signal from the quadrature signal using a discrete timesequence detector. An overview of the circuitry to implement thistechnique is shown in FIG. 4.

The diagonal signal S1 is generated by adding the output of thephotodetectors A+C, and the diagonal signal S2 is generated by addingthe output of photodetectors B+D. As described in greater detail belowwith reference to FIG. 10, the photodetector signals A, B, C and D aresampled and the addition operation implemented in discrete time as an ORoperation. Thus, the diagonal signals S1 and S2 in FIG. 4 are actuallydiscrete time signals S1(n) and S2(n). The diagonal signals S1 and S2are correlated using an adaptive dual arm correlator (ADAC) 13, thedetails of which are disclosed below. The ADAC 13 computes positive andnegative correlation signals CorrP 60A and CorrN 603 which are added P+N21 and subtracted P-N 20 to generate first and second position errorsignals (PES) which are in quadrature. The PES signals 20 and 21 arethen filtered by a bandpass/lowpass filter 23 to extract the trackcrossing harmonic, and the filtered signals processed by a discrete timesequence detector 22. The discrete time sequence detector 22 generates atrack crossing signal 24 by effectively filtering out spurious pulses inthe quadrature signal (the first and second PES signals P-N 20 and P+N21), and a servo controller (not shown) processes the track crossingsignal 24 to generate the appropriate servo control signals forcontrolling the motion of the sled assmebly 8 and OL carriage 11. At thebeginning and end of seeks, when the track crossing velocity is veryslow, the servo controller processes the filtered PES signals P-N 20 andP+N 21 directly since the output 24 of the sequence detector 22 is notvalid at this time.

Dual Arm Correlator

In the present invention, the dual arm correlation (DAC) is computed asthe difference between a positive correlation and a negative correlationof the diagonal signals S1(n) and S2(n) at a predetermined correlationoffset ##EQU1## where the first term represents the positive correlationand the second term represents the negative correlation. In the aboveequation, L is the length of the correlation which is carried out bysumming the XNOR (i.e., XOR denoted x) of the corresponding L-bits inthe binary signals S1(n) and S2(n).

FIG. 3A shows the waveforms corresponding to the positive correlationCorrP(A) and the negative correlation CorrN(Δ), as well as the resultingposition error signal (PES) computed as the difference between thepositive and negative correlations. The x-axis represents the phaseoffset between the diagonal signals S1 and S2, which is also theposition error. In this embodiment, the correlation offset A remainsfixed at approximately 1/4 the period of S1 and S2. Notice that themaximum range of PES in this embodiment is only plus or minusone-quarter of a track. If the correlation offset is not set to 1/4 theperiod of S1 and S2, or if the frequency of S1 and S2 drift due tovariations in angular velocity of the disk, then the maximum track rangefor PES is reduced even further. Furthermore, because the correlationoffset A is fixed, the positive and negative correlations are sensitiveto the frequency content of the recorded data which results inundesirable gain variance similar to the prior art differential phaseerror detectors described above.

Adaptive Dual Arm Correlator

In order to increase the range of the PES to one-half a track, as wellas compute the PES in a manner that is insensitive to the frequencycontent of the recorded data, the present invention employs an adaptivedual arm correlator (ADAC). The ADAC of the present invention adaptivelyadjusts the correlation offset Δ in order maximize the correlationbetween S1 and S2. In this manner, the instantaneous correlation offsetΔ represents the phase offset between the diagonal signals S1 and S2over a range of plus or minus one-quarter of a track, and the differencebetween the positive and negative correlations CorrP(Δ) and CorrN(Δ)represents the phase offset (i.e, PES) over a range extending to plus orminus one-half of a track as shown in FIG. 3B. Additionally, the ADAC ofthe present invention is substantially insensitive to the frequencycontent of the recorded data which allows for a higher servo trackingbandwidth due to a reduction in gain variance.

FIG. 4 shows a block diagram overview of the ADAC differential phaseerror detector 13 of the present invention. The quadrants A and C of thephotodetector are added to generate the diagonal signal S1, and thequadrants B and D are added to generate the diagonal signal S2. Thediagonal signals S1 and S2 are then input into a positive correlatorCorrP(Δ) 14L and a negative correlator CorrN(Δ) 14B, both of whichgenerate three correlation signals which are input into an ADACprocessor 16 for use in computing the updated correlation offset Δ18 asdescribed in more detail below with reference to the flow diagram ofFIG. 12.

The ADAC processor 16 computes two position error signals: PES 20 as thedifference between the positive and negative correlations P-N, and PES21 as the addition of the positive and negative correlations P+N. ThePES signals 20 and 21 are sinusoids ninety degrees out of phase;therefore, the PES signals 20 and 21 are in quadrature. The PES signals20 and 21 are bandpass or lowpass filtered 23 to extract the trackcrossing harmonic as described below with reference to FIG. 5, and thefiltered PES signals are input into a sequence detector 22 which detectsa track crossing signal 24 for use in counting the number of tracks theoptical transducer crosses during a seek operation. The discrete timesequence detector 22 effectively filters out spurious pulses in thequadrature PES signals 20 and 21 as described in more detail below.

During seek operations, the length L 26 of the correlations isprogrammably adjusted relative to the track crossing velocity such thatthe positive and negative correlators output a substantially constantnumber of correlations per track crossing. The following equationdescribes the desired correlation length: ##EQU2## where F_(s) is themaster clock frequency (system clock), F_(P-N) is the track crossingfrequency or velocity, and #SAMPLES is the desired number of samples inthe PES signals 20 and 21 per track crossing. The value selected for#SAMPLES determines the complexity and performance of the sequencedetector described below. In the preferred embodiment, #SAMPLES equalseight but it can be set higher or lower depending on the characteristicsof the storage device and the desired performance. A velocity detector28 detects the track crossing frequency F_(P-N) and adjusts thecorrelation length L(k) 26 according to the following equations:##EQU3## where F_(CLKS) is the number of 4·F_(S) /L clocks between everyother P-N zero crossing. Thus, the velocity detector 28 of FIG. 4implements the above error(k) equation by processing the P-N PES signal20 to detect zero crossings, counting the number of FCLKS between everyother P-N zero crossing, and subtracting the #SAMPLES expected between aP-N zero crossing. The first term in the above error(k) equation is asimple low pass averaging filter for averaging the detected number ofF_(CLKS) between every other P-N zero crossing; this filter can beadjusted to achieve different levels of performance.

Referring now to FIG. 10, shown is the preferred embodiment for thefront-end circuitry for sampling the photodetector signals A, B, C, andD, and generating the diagonal signals S1(n) and S2(n). A bandpassfilter 36 filters the photodetector signals to attenuate the DCcomponent (including the track crossing frequency) and to attenuatealiasing noise. The sinusoidal signals 38 output by the bandpass filter36 are converted into polarity square wave signals 40. The conversion isachieved by passing the sinusoidal signals 38 through polaritycomparators 42 that use hysteresis to prevent extraneous pulses in theoutput around the zero crossings. The polarity square waves are thensampled 44 and converted into binary square wave signals 46 that areshifted into 31-bit shift registers 48. The shift registers 48 allow theuser to selectively delay the photodetector signals with respect to oneanother in order to calibrate the servo control system according to theparticular characteristics of the storage device. The delay elements 50are implemented as a multiplexer for selecting the appropriate output ofthe shift registers 48 according to the delay value asserted overcontrol line 52. The diagonal signals S1(n) and S2(n) are then generatedby ORing 54 the binary square wave signals output by the delay elements50 to generate the signals (A+C) and (B+D), respectively. The diagonalsignals S1(n) and S2(n) are then input into the positive and negativecorrelators CorrP(Δ) 14A and CorrN(Δ) 14B of FIG. 4.

Further details of the positive and negative correlators CorrP(Δ) 14Aand CorrN(Δ) 14B are shown in FIG. 11. Part of the adaptive algorithm isto compute two correlations separated by a predetermined offset for eachof the positive and negative arms designated ±CorrP and ±CorrN. Thecorrelation offset Δ is then adaptively adjusted in a direction thatmaximizes the positive or negative correlation as described below. Thus,the positive correlator CorrP(Δ) 14A computes +CorrP according to acorrelation delay Δ+, and computes a positive correlation -CorrPaccording to a correlation delay Δ-, where Δ- is slightly smaller thanΔ+. Similarly, the negative correlator CorrN(Δ) 14B computes negativecorrelations +CorrN and -CorrN using the correlation offsets A+ and Δ-,respectively.

To compute the correlation signal +CorrP, the diagonal signal S1(n) isshifted undelayed into a first L-bit shift register 62; and the diagonalsignal S2(n) is shifted into a second L-bit shift register 62B afterbeing delayed by Δ+ 64. The length L of the shift registers determinesthe length of the correlation. The correlation signal +CorrP isgenerated by summing the XNOR 66 of the corresponding bits stored in theshift registers 62A and 62B, where XNOR (denoted x) is an inverted XORfunction ##EQU4## Similar circuitry is provided to generate thecorrelation signal -CorrP using Δ- as the correlation offset. Thecorrelation signals +CorrP and -CorrP are then added at adder 68 togenerate the positive correlation signal CorrP.

The negative correlator 14B for computing the negative correlationsignals +CorrN, -CorrN and CorrN comprises the same circuitry as thepositive correlator 14A of FIG. 11 except that S2(n) is undelayed andS1(n) is delayed by the correlation offsets Δ+ and Δ-.

In the preferred embodiment, the correlation is computed at a frequencyof once per L/4 bits shifted into the shift registers. That is, thecorrelation frequency is 4/L times the sampling frequency of thediagonal signals S1 and S2 such that each correlation is computed withL/4 new samples of S1(n) and S2(n). For tracking operations, the lengthand frequency of the correlation can be programmably adjusted in orderto optimize the phase error detector based on system dynamics such asthe linear velocity of the disk at a particular track.

The positive correlation signals (+CorrP, CorrP, -CorrP) 60A and thenegative correlation signals (+CorrN, CorrN, -CorrN) 60B are transferredto the ADAC processor 16 of FIG. 4 which computes the updatedcorrelation offsets Δ+ and Δ- according to the flow diagram shown inFIG. 12. The flow diagram of FIG. 12 updates the correlation offsets Δ+and Δ- in a direction that will maximize the positive or negativecorrelation values CorrP or CorrN. At step 70, the ADAC processor waitsin a loop for the next correlation period (i.e., when the samplingperiods t_(s) modulo divided by L/4 equals zero). Then at step 72, themagnitude of the positive and negative correlation signals CorrP andCorrN are compared. If CorrP is greater than CorrN, then at step 74 thecorrelation offset Δ is updated to Δ+ or Δ- according to the maximumbetween +CorrP and -CorrP. If CorrP is less than CorrN, then at step 76the correlation offset Δ is updated to Δ+ or Δ- according to the maximumbetween +CorrN and -CorrN. Also at steps 74 and 76, the phase offset ΔΘbetween the diagonal signals S1(n) and S2(n) is saved and used tocompute the quadrature signal for seeking as described below. At step78, the correlation offsets Δ+ and Δ- are updated to the current valueof Δ plus and minus a predetermined offset Δ_(MIN). If the updatedcorrelation offsets are out of range, then at step 80 they are adjustedto a maximum or minimum value as necessary. Finally, the first andsecond position error signals (PES) 20 and 21 are computed as thedifference and sum of CorrP and CorrN at step 82.

Referring again to FIG. 4, the position error signals (PES) 20 and 21are bandpass/lowpass filtered 23 to extract the track crossing harmonic.During the seek, the track crossing harmonic is extracted with abandpass filter when the tracking crossing velocity is above apredetermined threshold. As the read head approaches the target trackand the track crossing velocity decreases, eventually the correlationlength L will reach a maximum value. At this point, the track crossingharmonic is extracted with a lowpass filter which simply attenuates highfrequency noise. Thus, the correlation length L is compared to a maximumvalue, and the result of the comparison selects the output of thebandpass or lowpass filter through multiplexers as shown in FIG. 5. Atthe beginning and end of seeks, when the track crossing velocity is veryslow, the servo controller processes the filtered PES signals P-N 20 andP+N 21 directly since the output 24 of the sequence detector 22 is notvalid when the correlation length L saturates (reaches its maximumvalue).

The bandpass and lowpass filters of FIG. 5 are discrete time filtersclocked at the output frequency of the correlators. Consequently, thespectrum of the bandpass filter automatically scales to the trackcrossing harmonic since the output of the correlators are matched to thetrack crossing frequency as described above. In other words, it is notnecessary to program the coefficients of the bandpass filter to matchits spectrum to the track crossing harmonic; this is done automaticallywhen the correlator output frequency changes by adjusting thecorrelation length L.

Trellis Sequence Detector

The present invention employs a Viterbi type maximum likelihood trellissequence detector 22 for detecting a track crossing signal 24 from thequadrature position error signals (PES) 20 and 21 (after filtering 23).After extracting the track crossing harmonic, the position error signalsare sinusoidal with a 90 degree offset from one another and withrespective amplitudes of ±A and ±AA as shown in FIG. 6. As describedabove, the correlation length L is adjusted relative to the trackcrossing velocity so that the correlators 14A and 14B output apredetermined number of samples per track crossing. In the preferredembodiment, the correlators are designed to output eight correlationsamples per track crossing, but this is not a limiting aspect of thepresent invention. As described above, the number of correlation samplesper track crossing can be increased or decreased depending on thedesired performance versus complexity of the system.

The correlation samples per track crossing are associated with targetsamples of the sinusoidal PES signal. For example, the eight correlationsamples in the preferred embodiment are associated with eight targetsamples in the sinusoidal PES signals shown in FIG. 6. The eight targetsamples (0-7) in each PES signal correspond to eight states of a statetransition diagram shown in FIG. 7A. Each state in the state transitiondiagram of FIG. 7A corresponds to a phase location on the sinusoidalsignals of FIG. 6. The first state, state 0, corresponds to a 0 samplevalue in the first PES sinusoid and to a -AA sample value in the secondPES sinusoid as shown in FIG. 6. The next state, state 1, corresponds toa +0.71A sample value in the first PES sinusoid and to a -0.71AA samplevalue in the second PES sinusoid, and so on. The transition branchesleading from one state to another are labeled with the input samplesthat would lead to a particular next state. For example, if the currentinput samples correspond to state 0, and the next input samples are(0.71A, -0.71AA), then a transition occurs to state 1. As shown in FIG.7A, the state transition sequences are contiguous due to the periodicnature of the data. The only two possible state transition sequences arein a clockwise or counter clockwise direction depending on whether thestorage system is executing a forward or reverse seek.

The trellis sequence detector 22 of FIG. 4 operates according to thestate transition diagram of FIG. 7A. To understand the operation, thestate transition diagram is represented as a trellis, part of which isshown in FIG. 7B. The trellis in FIG. 7B represents a time sequence ofinput samples relative to the target samples shown in FIG. 6. Thebranches between the states represent the squared error between theinput sample and the target sample which is referred to as an errormetric. The various paths through the trellis represent the possiblevalid sequences associated with the target sample sequence of FIG. 6.The sequence detector 22 computes a most likely valid sequenceassociated with the input sample sequence by computing a squared errorbetween the valid sequences and the input sample sequence. The validsequence closest to the input sample sequence (in Euclidean space) isselected as the output of the sequence detector 22.

A number of candidate sequences are generated as the input samples areprocessed. The candidate sequences are saved in what is known as "pathmemories" of the detector. Notice that each state in the trellis of FIG.7B can be reached from three different paths; however, only one of thethree paths will be the most likely path, that is, the path with thesmallest accumulated error metric. Thus, the most likely path isselected and the remaining paths are discarded. The candidate sequencesassociated with the discarded paths merge into the survivor sequencesthat comprise the paths that were selected. Eventually, all of thecandidate sequences will merge into a single most likely sequence whichbecomes the output sequence. The number of candidate sequences that mustbe saved in the path memories equals the number of states in the trellis(i.e., eight). The path memories are designed to be long enough toensure that the paths merge (which is a function of the signal-to-noiseratio (SNR)), but also to be as short as possible to reduce latency andcircuitry cost.

A block diagram of the trellis sequence detector 22 and its componentsis shown in FIG. 8. The samples of the filtered quadrature signals P+Nand P-N are input into a metric generator 30 which computes an errormetric for each of the three branches into each of the eight states ofFIG. 7A. In the preferred embodiment, the error metric is computed asthe squared error between the actual sample values of the quadraturesignals P+N and P-N and the target sample values shown in FIG. 6. Inother words, the trellis sequence detector computes the most likelyquadrature signal in Euclidean space. The 24 error metrics 32 areprocessed by a branch selector 34 which selects the branch into each ofthe eight states that has the smallest accumulated error metric. Foreach branch selected, a corresponding target sample value of FIG. 6 isloaded into the eight path memories 35₀ -35₇. The branch selector 34also merges the sequences stored in the path memories 35₀ -35₇ whichcorrespond to the branches not selected. Eventually, all of thesequences stored in the path memories 35₀ -35₇ will merge into a singlemost likely sequence which is selected as the output 24 of the trellissequence detector. In FIG. 8, the output 24 of the trellis sequencedetector is selected as the output of the last path memory; however, theoutput could be selected from any of the path memories since they allstore the same merged survivor sequence. As explained above, the pathmemories are designed to be long enough to guarantee that the pathsmerge into a single survivor sequence, but also as short as possible tominimize the latency and circuitry cost.

The trellis sequence detector 22 outputs a unique discrete value foreach state in the state transition diagram of FIG. 7A.

Thus, the detected track crossing signal 24 output by the sequencedetector 22 is a staircased, periodic sawtooth wavefrom as shown in FIG.9A and FIG. 9B. The wavefrom of FIG. 9A corresponds to a forward seek,and the waveform of FIG. 9B corresponds to a reverse seek. The trackcrossing signal 24 is processed by the servo control circuitry todetermine the direction of movement and number of tracks crossed duringa seek operation.

The performance enhancement provided by the trellis sequence detector 22is to filter out spurious pulses in the quadrature PES signals (20 and21) that can occur due to noise in the system. The sequence detectoreffectively replaces erroneous samples in the PES signals with validsamples that correspond to the target samples of the expected sinusoidalsignals shown in FIG. 6. This improves the overall performance of theservo system during seeks by reducing the number of misdetected trackcrossings and/or missed track crossings.

The objects of the invention have been fully realized through theembodiments disclosed herein. Those skilled in the art will appreciatethat the various aspects of the invention can be achieved throughdifferent embodiments without departing from the essential function. Forexample, the desired number of correlation samples per track crossingcould be adjusted to achieve a varying degree of performance versuscircuit complexity. As the desired number of correlation samplesincreases, the performance increases but the complexity also increasesdue to the increased number of states in the sequence detector. Theparticular embodiments disclosed are illustrative and not meant to limitthe scope of the invention as appropriately construed by the followingclaims.

We claim:
 1. In an optical storage device for recording digital data, aservo system for controlling the location of a light-beam reflected offof an optical storage medium, the light-beam generated by an opticaltransducer, the servo system comprising:(a) a quadrature signalgenerator for generating a discrete time quadrature signal indicative ofthe light-beam crossing data tracks recorded on the optical storagemedium during a seek operation; and (b) a trellis sequence detector fordetecting a track crossing signal from the discrete time quadraturesignal by computing an estimated quadrature signal relative to samplevalues of the discrete time quadrature signal and target sample valuesof an ideal quadrature signal.
 2. The servo system as recited in claim1, wherein the trellis sequence detector comprises:(a) an error metricgenerator for generating an error metric as a function of a sample valueof the discrete time quadrature signal and a target sample value of theideal quadrature signal; (b) a plurality of path memories for storingsequences of ideal sample values of the ideal quadrature signal, thesequences of ideal sample values representing paths through a trellisdiagram; and (c) a branch selector for selecting a trellis branch basedon the error metric and for merging the sequences stored in the pathmemories based on the selected trellis branch.
 3. The servo system asrecited in claim 1, wherein the discrete time quadrature signalcomprises first and second discrete time sinusoidal signals phase offsetby 90 degrees.
 4. The servo system as recited in claim 1, wherein thequadrature generator comprises a controller for controlling the numberof samples in the discrete time quadrature signal per track crossingrelative to the track crossing velocity of the light-beam.
 5. The servosystem as recited in claim 4, wherein the controller comprises:(a) acorrelator for correlating position signals indicative of a location ofthe light-beam with respect to a centerline of a data track; (b) avelocity detector for detecting a track crossing velocity of thelight-beam; and (c) a correlation length generator for generating acorrelation length based on the track crossing velocity, the correlationlength input into the correlator.
 6. The servo system as recited inclaim 2, wherein the quadrature generator comprises a controller forcontrolling the number of samples in the discrete time quadrature signalper track crossing relative to the track crossing velocity of thelightbeam.
 7. The servo system as recited in claim 6, wherein the numberof samples in the discrete time quadrature signal per track crossingequals a number of states in the trellis diagram.
 8. The servo system asrecited in claim 6, wherein the controller comprises:(a) a correlatorfor correlating position signals indicative of a location of thelight-beam with respect to a centerline of a data track; (b) a velocitydetector for detecting a track crossing velocity of the light-beam; and(c) a correlation length generator for generating a correlation lengthbased on the track crossing velocity, the correlation length input intothe correlator.
 9. The servo system as recited in claim 5, wherein:(a)the position signals are discrete-time signals S1(n) and S2(n); and (b)the correlator computes a correlation corr (Δ) according to ##EQU5##where L is the correlation length and Δ is an integer representing atime shift between S1(n) and S2(n).
 10. The servo system as recited inclaim 8, wherein:(a) the position signals are discrete-time signalsS1(n) and S2(n); and (b) the correlator computes a correlation corr(Δ)according to ##EQU6## where L is the correlation length and Δ is aninteger representing a time shift between S1(n) and S2(n).
 11. In anoptical storage device for recording digital data, a servo system forcontrolling the location of a light-beam reflected off of an opticalstorage medium, the light-beam generated by an optical transducer, theservo system comprising:(a) a servo signal generator for generating adiscrete time servo signal indicative of the light-beam crossing datatracks recorded on the optical storage medium during a seek operation;and (b) a trellis sequence detector for detecting a track crossingsignal from the discrete time servo signal by computing an estimatedservo signal relative to sample values of the discrete time servo signaland target sample values of an ideal servo signal.
 12. The servo systemas recited in claim 11, wherein the discrete time servo signal is adiscrete time quadrature signal comprising first and second discretetime sinusoidal signals phase offset by 90 degrees.
 13. The servo systemas recited in claim 12, wherein the trellis sequence detectorcomprises:(a) an error metric generator for generating an error metricas a function of a sample value of the discrete time quadrature signaland a target sample value of the ideal quadrature signal; (b) aplurality of path memories for storing sequences of ideal sample valuesof the ideal quadrature signal, the sequences of ideal sample valuesrepresenting paths through a trellis diagram; and (c) a branch selectorfor selecting a trellis branch based on the error metric and for mergingthe sequences stored in the path memories based on the selected trellisbranch.
 14. The servo system as recited in claim 12, wherein the servosignal generator comprises a controller for controlling the number ofsamples in the discrete time quadrature signal per track crossingrelative to the track crossing velocity of the light-beam.
 15. In anoptical storage device for recording digital data, a servo controlmethod for controlling the location of a light-beam reflected off of anoptical storage medium, the light-beam generated by an opticaltransducer, the servo control method comprising the steps of:(a)generating a discrete time servo signal indicative of the light-beamcrossing data tracks recorded on the optical storage medium during aseek operation; and (b) detecting a track crossing signal from thediscrete time servo signal by computing an estimated servo signalrelative to sample values of the discrete time servo signal and targetsample values of an ideal servo signal.
 16. The servo control method asrecited in claim 15, wherein the discrete time servo signal is adiscrete time quadrature signal comprising first and second discretetime sinusoidal signals phase offset by 90 degrees.
 17. The servocontrol method as recited in claim 16, wherein the step of detecting thetrack crossing signal comprises the steps of:(a) generating an errormetric as a function of a sample value of the discrete time quadraturesignal and a target sample value of the ideal quadrature signal; (b)storing sequences of ideal sample values of the ideal quadrature signalin a plurality of path memories, the sequences of ideal sample valuesrepresenting paths through a trellis diagram; (c) selecting a trellisbranch based on the error metric; and (d) merging the sequences storedin the path memories based on the selected trellis branch.
 18. The servocontrol method as recited in claim 16, further comprising the step ofcontrolling the number of samples in the discrete time quadrature signalper track crossing relative to the track crossing velocity of thelight-beam.